1. Field of the Invention
The invention relates to testing of electrochemical sensors used for gas detection.
2. Description of Related Art
Electrochemical sensors are widely used in detecting oxygen (O2) and a variety of toxic gases such as carbon monoxide (CO), hydrogen sulfide (H2S), hydrogen dioxide (SO2), nitrogen dioxide (NO2), nitric oxide (NO), chlorine (Cl2), chlorine dioxide (ClO2), ozone (O3), hydrogen (H2) and ammonia (NH3). These sensors typically comprise two or three electrodes separated by an acid or basic electrolyte, and are usually operated in amperometric mode. In operation, the working electrode potential is controlled with respect to the potential of a reference or counter electrode and the current output due to reaction of a gas at the working electrode is monitored. Electrochemical sensors need very little power to operate, are both sensitive, accurate, and can be made specific to gases of interest. However, electrochemical sensors may fail silently, i.e. without indication, for a number of reasons, including degradation of catalyst electrodes, leakage or drying out of electrolyte and broken or corroded metal pins, etc. Because electrochemical sensors are widely used in many critical applications include medical devices, safety products for protection of personnel against potentially harmful atmospheres and emission monitoring, the unrecognized failure of a sensor can have disastrous consequences. A reliable sensor diagnostic system would be a major advantage for gas detection instruments.
There are a number of patents on sensor diagnostic methods. Generally speaking, the methods fall into two major categories, electronic methods and gas test methods.
Electronic methods are disclosed, for example, in U.S. Pat. Nos. 5,202,637, 5,558,752 and 6,251,243, in which a pulse or alternating voltage of small amplitude is applied between the working electrode and the reference electrode. If the resulting current flowing through the sensor is greater than a predetermined threshold value, then the sensor is considered to be functioning correctly. Other electronic tests include those disclosed in U.S. Pat. No. 6,088,608, which describes electronic circuitry for automatically performing an integrity test based on impedance measurements, U.S. Pat. No. 6,200,443, which describes a sensor diagnostic method based on measuring capacitance by applying a small voltage, e.g. 10 mV of 10 second duration, across a sensor and measuring the rate of change of current output when the voltage is removed, and U.S. Pat. No. 6,049,283 which describes a method of detecting a fault condition by monitoring noise level from sensor output. Low noise is an indication that the sensor may have a broken wire or contact or the electrolyte may have dried out.
Gas test methods are disclosed, for example, in U.S. Pat. Nos. 5,668,302 and 6,200,443 and Japanese Publication No. 11-083792, in which a gas sensor assembly includes a gas generator which supplies a test gas, usually hydrogen, directly to the sensor on a regular basis to cause a response in the sensor. The instrument monitors the sensor""s response to this test gas to determine if the sensor works properly.
Electronic methods have been widely used in gas detection instruments. A major advantage of these methods is that they can be performed automatically in an instrument on a scheduled basis. The impedance properties of electrochemical sensors can be modeled by networks of resistors and capacitors. Electronic diagnostic methods can not only check the integrity of the sensor and associated sensor control circuitry, but also determine if there is a significant reduction in the capacitance of the electrode/electrolyte interfaces, and/or a decrease in the conductivity of electrolyte. Both of these changes can reflect low gas sensitivity due to greatly reduced catalytic activity of electrodes, or insufficient electrolyte in the sensor cell.
However, electronic tests as described above can not detect if there is a leakage of electrolyte, and/or if the gas access hole is blocked. While a very low capacitance is often indicative of a low sensitivity, a large capacitance does not necessarily mean high sensor sensitivity. For example, most electrochemical sensors employing an acid electrolyte tend to have electrolyte leakage after having been exposed to high humidity for an extended period of time. Prior to electrolyte leakage occurring, the hygroscopic electrolyte absorbs a substantial amount of water from the surrounding atmosphere, resulting in an increase in electrolyte volume and eventually a high-pressure buildup inside the sensor. The high pressure forces some of the electrolyte into the working electrode beyond the amount normally present. The overall capacitance of the working electrode increases, but the sensor sensitivity becomes very low because most of the active catalyst sites in the electrode are flooded and gas diffusion into these sites becomes more difficult.
Gas test methods appears to be more reliable than electronic methods. By using a real gas, usually hydrogen (H2), in the test, the method is capable of determining if the sensor is sensitive enough to a gas, and if the gas generator is located outside the sensor, this method can detect if sensor sampling hole is plugged. However, the gas concentration is difficult to control due to varying environmental conditions. Moreover, the gas generator is an electrolytic cell itself and may fail well before the sensor, since it relies on electrolysis of water to generate hydrogen gas. This method is only applicable to hydrogen and CO sensors; O2 sensors and most toxic gas sensors do not respond to H2 at all.
It is therefore the object of the invention to provide a simple and reliable method for diagnosing the condition of a gas sensor.
It is another object of the invention to enable detection of common sensor fault conditions, such as short circuits, broken electrical contacts, insufficient electrolyte, corroded metal pins and insufficient gas access.
It is a further object of the invention to detect if a sensor has sufficient sensitivity to detect an intended gas.
To achieve these and other objects, the invention is directed to a method for determining sensitivity of an electrochemical gas sensor including at least a working electrode and a reference electrode, and a diffusion limited inlet, comprising the steps of:
connecting the sensor to potentiostat means and means for determining output current;
establishing a base output current;
causing a sudden change in water vapor pressure at the inlet, maintaining said changed water vapor pressure over a period of at least several seconds, and recording a resultant change in output current; and
comparing the change in output current to a standard to determine sensor sensitivity.
Almost all electrochemical gas sensors are operated in such a way that the working electrode potential is held sufficiently positive to oxidize an analyte gas, or sufficiently negative to reduce the gas. In the presence of a reactive gas, the current output of a sensor depends on the flux of gas through the gas diffusion path, and the number of active catalyst sites available for electrochemical reaction on the working electrode. Gas access holes, dust membranes, gas filters, and the backing support of the working electrode are all common gas diffusion barriers in electrochemical sensors. Assuming the sensor responds to the presence of a second gas, either directly or indirectly, then the second gas can be used as a reference to test the functional status of the sensor. Generally speaking, the higher the sensor""s sensitivity to the second gas, the higher the sensor""s sensitivity to the target gas.
Most currently used electrochemical gas sensors employ an aqueous electrolyte such as diluted sulfuric acid (H2SO4), and a working electrode made of a precious metal catalyst such as platinum (Pt). The potential of the working electrode is governed by the redox couple of oxygen (O2) and water (H2O) in clean air. When an equilibrium is established between oxidation of water and reduction of oxygen, the working electrode becomes stable in potential. However, if there is a sudden change in the atmosphere that affects the electrochemical reactions, the equilibrium will be broken and the potential of the working electrode will change, causing a potential difference between the working electrode and the reference electrode; as a result, current flows through the sensor.
According to the invention, a sharp change in water vapor pressure, or more generally, relative humidity at a given temperature, is made at the inlet to an electrochemical gas sensor. Because of its hygroscopic properties, the electrolyte absorbs or loses water, which causes a sharp change in the acidity at the working electrode, and hence, a current response from the sensor. Because the change in water vapor pressure takes place outside the sensor housing, the sensor""s response to this vapor pressure change is very similar to its response to the entry of a reactive gas to the sensor. If the response to the pressure change is too small or there is no response at all, the sensor will not be sensitive enough to detect any other reactive gas.
The change in water vapor pressure, or relative humidity, necessary to cause a measurable sensor response is thought to be about 5%. The larger the change, the greater the response of the sensor. The source of the change in water vapor pressure may, in fact, be air in either damp or dry areas, ambient air that has passed through a desiccator or a humidifier, or a tank of compressed air, which is sometimes called xe2x80x9cdry airxe2x80x9d and is extremely low in water content. The air which is tested as a result, however, will be air of xe2x80x9cambientxe2x80x9d composition except for a change in water vapor pressure.
The change in water vapor pressure at the inlet to the sensor must be conducted in a reproducible manner, with a properly functioning sensor then delivering a reproducible change in output current. The normal change in output current will depend on the sensor itself, and will be determined from test results from a large number of sensors of a particular type, and the desired sensor sensitivity. Advantageously, a normal value, e.g. xe2x88x9250 nA, will be stored in the instrument as a threshold value for comparison with the test value. If the test value is insufficient, the instrument can be taken out of service and examined for defects. Alternatively, the sensor response can be compared with a previous response in the same test, and the sensor will be considered to be failing if there is a significant decrease in the output value.
The invention is also directed to an apparatus for carrying out the method, including a diffusion limited electrochemical gas sensor cell, means for causing a sudden change in water vapor pressure at the inlet of the cell, and means for connecting the cell inlet with the vapor pressure changing means. Preferably, the apparatus will also include control means, including potentiostat means for operating the cell, means for determining and/or recording cell output current, means for controlling the duration of the change in water vapor pressure, and means for comparing the change in output current with a normal value. Some of the control and/or recording means may be at a remote location, with control by telephone, wireless communication or computer network.